moo is a game where the user has to guess a number choosen by the computer.
By default,
the computer takes a number of four digits (including 0's),
but that can be changed by giving moo the number of digits to take.
After each guess,
the number of bulls and cows is displayed.
A bull is a correctly guessed digit,
in the right place,
while a cow is a correct digit,
not in the right place.
Once a game has finished because all the digits have been guessed correctly,
a new game will be started.
Exiting the program can be done by typing 'q' or 'Q' on a guess,
or hitting the interrupt key (usually control-C).